The invention relates generally to a capsule to be used with pressure vessels. More particularly, the invention relates to a capsule used in conjunction with a high-pressure vessel for processing at least one material in a supercritical fluid.
Supercritical fluids (also referred to hereinafter as “SCF”) may be used to process a wide variety of materials. Examples of SCF applications include extractions in supercritical carbon dioxide, the growth of quartz crystals in supercritical water, and the synthesis of a variety of nitrides in supercritical ammonia.
Processes that employ supercritical fluids are generally performed at high pressure and high temperature (also referred hereinafter as “HPHT”) within a pressure vessel. Most conventional pressure vessels not only provide a source of mechanical support for the pressure applied to reactant materials and SCF, but also serve as a container for the supercritical fluid and material being processed. The processing limitations for such pressure vessels are typically limited to a maximum temperature in the range between about 550° C. and 750° C. and a maximum pressure in the range between about 2 kilobar (also referred hereinafter as “kbar”) and 5 kbar.
Processing material with supercritical fluids requires a container or capsule that is both chemically inert and impermeable to the solvent and any gases that might be generated by the process. In one approach, the material to be processed, along with a solid or liquid that forms a supercritical fluid at elevated temperatures, is introduced into a capsule. The capsule is then sealed in air, placed in a high pressure apparatus, and heated. The solid (or liquid) decomposes upon heating to provide a supercritical fluid. When such a solid or liquid is used as the SCF source, however, decomposition products other than the supercritical fluid that remain in the reaction mixture may contaminate the reaction mixture. Additional contamination may also result from air introduced during filling of the capsule.
In one method, air may be excluded from a capsule by placing the material to be processed into a fused silica tube having a closed end, evacuating the tube through a vacuum manifold, and condensing a solvent into the tube. The tube is then sealed, usually by welding, without exposing the contents of the capsule to air. Once the capsule is sealed, however, the material inside the tube cannot be processed at internal pressures greater than about 6 bar and temperatures higher than about 300° C., as the internal pressure generated by vaporization of the solvent will cause the sealed capsule to burst when heated to higher temperatures. An external pressure greater than or equal to the internal pressure can be provided by placing the capsule inside a pressure vessel and filling the space between the capsule and the pressure vessel with a solvent. However, as noted above, such pressure vessels are typically limited to a maximum temperature in the range between about 550° C. and 750° C. and a maximum pressure in the range between about 2 and 5 kbar.
If the pressure, temperature, chemical-inertness, size, sealing, and cost limitations of currently available capsules could be extended, supercritical fluids could be used to process a wider range of materials. Therefore, what is needed is an improved capsule or container for processing of materials with supercritical fluids in an air-free environment. What is also needed is a capsule that can be utilized with a solvent that is gaseous at room temperature. What is further needed is a chemically inert capsule that may be used in conjunction with a pressure vessel that is capable of generating pressures greater than about 5 kbar and temperatures between about 550° C. and about 1500° C. What is further needed is a chemically inert capsule that can cost-effectively process materials on a larger scale.